1. Field of the Invention
The present invention relates to an internal combustion engine.
2. Description of the Related Art
Known in the art is a compression-ignition type engine wherein fuel is injected into the combustion chamber in a compression stroke slightly before 60 degrees before top dead center in the compression stroke, the mean particle size of the fuel injected at that time is made at least a particle size where the temperature of the fuel particles reaches a boiling point of the main fuel ingredient, determined by the pressure at that time, at about top dead center in the compression stroke or after top dead center in the compression stroke so as to prevent evaporation of the fuel due to boiling of the main fuel ingredient from the fuel particles after injection up until the top dead center of the compression stroke is reached and so as to make the main fuel ingredient in the fuel particles boil and evaporate to cause ignition and combustion of the fuel after substantially the top dead center in the compression stroke (Japanese Unexamined Patent Publication (Kokai) No. 7-317588).
In this compression-ignition type engine, the intention is to make the particles of the injected fuel disperse uniformly in the combustion chamber and thereby to make the generation of soot and NO.sub.x substantially zero. That is, if the pressure in the combustion chamber becomes high, the air resistance becomes larger, so it is not possible to make the particles of the injected fuel disperse throughout a broad range. Therefore, to enable the particles of the injected fuel to be dispersed throughout a broad range, the fuel is made to be injected slightly before 60 degrees before top dead center of the compression stroke, where the pressure in the combustion chamber is low.
Even if the particles of the injected fuel are dispersed over a broad range, however, if a portion of heavy density of the evaporated fuel is formed from the fuel particles, NO.sub.x and soot will be generated. The reasons for this will be explained in detail later, but roughly speaking are as follows:
That is, if the fuel is injected slightly before 60 degrees before top dead center of the compression stroke in the above way and the fuel particles are large in size, the evaporation of fuel due to boiling of the main fuel ingredient of the fuel particles will be prevented until the substantially top dead center of the compression stroke is reached. The injected fuel, however, includes low boiling point ingredients. The boiling temperature of the low boiling point ingredients, that is, the initial boiling point, is a temperature considerably lower than the boiling temperature of the main fuel ingredient. Therefore, if the temperature in the combustion chamber at the time of fuel injection is higher than the initial boiling point, the low boiling point ingredients in the injected fuel will immediately evaporate. As opposed to this, if the temperature in the combustion chamber at the time of fuel injection is lower than the initial boiling point, the low boiling point ingredients in the injected fuel will evaporate when the temperature in the combustion chamber exceeds the initial boiling point. If the low boiling point ingredients in the injected fuel are evaporated, a layer of evaporated fuel comprised of the low boiling point ingredients will be formed around the fuel particles.
On the other hand, when the compression stroke proceeds, the temperature in the combustion chamber rises. When the temperature in the combustion chamber reaches a certain temperature or more, the evaporated fuel around the fuel particles will bond with oxygen and be burned. If the density of the fuel particles at this time is high, the fuel particles will receive the heat of combustion of the evaporated fuel from the surrounding fuel particles and become high in temperature. As a result, the hydrocarbons in the fuel particles will be decomposed by the heat into hydrogen atoms H.sub.2 and carbon C. The hydrogen atoms H.sub.2 produced by this heat decomposition will burn explosively and generate a high temperature, therefore NO.sub.x will be produced. On the other hand, if carbon C is produced due to the heat decomposition, the carbon atoms will bond with each other and will partially be exhausted as soot.
In this way, if the density of the fuel particles is high, NO.sub.x and soot will be produced due to the heat decomposition of the hydrocarbons in the fuel particles. To prevent the generation of such NO.sub.x and soot, it is sufficient to increase the interval between the fuel particles. Therefore, it is sufficient to cause the fuel particles to disperse over a wide range. Accordingly, the above in-cylinder injection type internal combustion engine is designed to inject the fuel slightly before 60 degrees before top dead center of the compression stroke so as to cause the fuel particles to disperse over a wide range.
The temperature in the combustion chamber slightly before 60 degrees before top dead center of the compression stroke, however, becomes higher than the initial boiling point of the injected fuel, so if fuel is injected, the low boiling point ingredients in the injected fuel will immediately evaporate. As a result, a region of high density evaporated fuel of the low boiling point ingredient will be locally formed around the fuel injector. If a region of high density evaporated fuel is formed in this way, even if the space between fuel particles becomes large, the fuel particles will become high in temperature due to the heat of combustion of the evaporated fuel present at a high density between the fuel particles and therefore the hydrocarbons in the fuel particles will be decomposed by the heat into hydrogen atoms H.sub.2 and carbon C, so NO.sub.x and soot will be generated.
To prevent the generation of such NO.sub.x and soot, it is necessary to prevent the local formation of a region of high density evaporated fuel of the low boiling point ingredient. Toward this end, it is necessary to complete the fuel injection and complete the dispersion of the injected fuel before the temperature in the combustion chamber reaches the initial boiling temperature.